The present invention relates to an amplifier for use in distributing video and other electrical signals. More particularly, the present invention relates to a method and circuit for increasing the output impedance of an inactive amplifier for use in transmitting such signals through a transmission line.
In the distribution of video and other electrical signals, it is often desirable to use a single transmission line (e.g., a coaxial cable or parallel conductors) to transmit a plurality of signals. Typically, a plurality of amplifiers will be coupled in parallel to such a line to transmit respective signals. In such distribution systems, it is desirable to be able to couple a large number of amplifiers to the transmission line in order to reduce the cost and complexity of distributing a plurality of signals to a remote location.
Generally, when a plurality of amplifiers are coupled in parallel to a single transmission line, the loading effect of the amplifiers on the transmission line must be taken into account. For example, while a particular amplifier is active and coupling a signal to the transmission line, it is desirable that the remaining amplifiers commonly coupled to the same transmission line do not disturb such coupling (i.e., the back termination of the transmission line). In other words, it is desirable that the remaining amplifiers do not alter the impedance of the load which the coupling amplifier drives.
In particular, the output impedance of an active amplifier and a series back-termination resistor is generally matched to the characteristic impedance of the transmission line in order to minimize reflection of power at the transmission line interface. Typically, since the output impedance of an active amplifier is substantially less than one ohm, the back-termination resistor is normally chosen to have a resistance that substantially matches the characteristic impedance of the transmission line. Such impedance matching increases the efficiency of the transmission system.
However, if the remaining amplifiers alter the impedance of the load (i.e., the back termination of the transmission line) which the coupling amplifier drives, then the series combination of the active coupling amplifier and back-termination resistor will no longer be matched to its load. This results in inefficiency and, in a worst case, can prohibit transmission altogether. Therefore, it is desirable that the remaining inactive amplifiers have as high an output impedance as possible under such conditions when another amplifier is active.
In light of the above, it can be seen that an ideal amplifier would have an infinite output impedance when the amplifier is inactive. If it were possible to have an ideal system including a plurality of ideal amplifiers coupled to a single transmission line, the total number of amplifiers which may be coupled together would generally then be unlimited.
However, the output impedance of amplifiers do not behave in the above-described ideal manner. The output impedance during inactive operation is generally not infinite. Thus, the impedance of the back termination that an active amplifier drives is generally altered by the presence of the remaining amplifiers on the transmission line. This loading effect generally limits the number of amplifiers that can be coupled in parallel to a single transmission line.
In view of the foregoing, it would be desirable to provide a method for allowing the number of amplifiers coupled in parallel to a single transmission line to be increased.
It would also be desirable to provide a method and circuit for increasing the output impedance of an inactive amplifier for use in distributing video and other electrical signals.
It would further be desirable to provide an amplifier having high output impedance in its inactive state that allows a plurality of such amplifiers to be coupled in parallel to a single transmission line with reduced signal degradation.
It would additionally be desirable to provide a system for coupling a plurality of amplifiers to a single transmission line wherein the feedback networks associated with each amplifier do not significantly load the transmission line.